Measurement integrity and reproducibility are important, but when companies select production test equipment, they are likely to be concerned with productivity issues, according to a study of electronic component manufacturers conducted by Keithley Instruments.
The study found instrument accuracy is taken almost as an article of faith, and production engineers quickly turn their attention to throughput, flexibility, equipment simplicity, test programming, footprint and reliability.
There is a tradeoff between speed and accuracy when making rapid multiple measurements with most instruments. One reason is that it takes time for a scanning relay’s switch contacts to settle. Instrument manufacturers can reduce this time by using electronic relays or mercury wetted contacts that have little or no bounce.
There also is settle time associated with the A/D converter and amplifier circuitry in an electronic instrument. One solution Keithley provides is a servo amplifier front end on its 2000 Series DMMs. The circuitry provides extremely low offset drift and autozeroing to speed up measurements.
To reduce electronic bottlenecks, an instrument’s dynamic measurement range can be broadened. This minimizes range shift errors and increases throughput by reducing range change delays.
Some manufacturers, such as Keithley, provide added flexibility by designing an instrument with programmable settle time. This allows you to decide between accuracy and speed; for instance, you could select 1,000 readings/s at 4 1/2 digits of resolution or 500 readings/s at 5 1/2 digits.
Instrument manufacturers are looking at how other system components can be more easily integrated with their products. One goal is to make it easier for the user to set up and program instruments and the switching system so they work together more effectively.
Instrument design considerations include flexible trigger I/O, ability to handle a wide range of signal types and levels, multiple electronic connectors, compatible data communication protocols, and the ability to stack or combine diverse instruments with rack mounting arrangements.
In Keithley’s 2000 Series DMMs, for example, front and rear panel I/O is designed to facilitate system integration using industry standard connections for test inputs and data communications. Besides IEEE 488 and RS-232 interfaces, these DMMs use the SCPI command set to provide flexible control of measurements over a data communications network. However, they can operate as self-contained multipoint measurement systems through use of a 10-channel scanner card.
By combining the DMM with Keithley’s Model 2400 Digital SourceMeter™, you get a measurement system for precision, high-speed tests requiring a broad range of source voltages or currents. The 2400 is designed for high-speed DC tests that simultaneously source and measure voltage, current or resistance. Its compact, single-box design saves space and simplifies system integration.
Often, production test equipment operates around the clock, seven days a week, under harsh manufacturing conditions. To provide reliable test capacity in this environment, instrument designers are improving the physical packaging of instrument circuits in key areas.
The Keithley 2000 Series DMMs, for example, make extensive use of surface mount technology, and circuit boards have been designed with no more than two layers, which holds down manufacturing costs. The Model 2010 Low Noise Multimeter has 7 1/2 digit resolution, yet maintains the throughput needed for production testing. It provides up to 2,000 readings/s at 4 1/2-digit resolution. In addition, it provides a 20 mV clamp for dry circuit testing of contact resistance. To avoid device self-heating, ohms can be measured at 100 µA. The Model 2010 costs $2,395.
The Model 2400 Digital SourceMeter also offers throughput advantages. Compared to systems created with separate instruments, it takes half the time to set up and program and uses one-fourth the typical rack space. Also, with speeds up to 1,000 readings/s, test time can be cut in half. The Model 2400 costs $3,495. Keithley Instruments, (800) 552-1115.
Copyright 1996 Nelson Publishing Inc.